How the Undecided Are Decisive

By

Peter A. Brown

Updated Oct. 28, 2010 12:01 a.m. ET

In the home stretch of tight political races, it gets harder to pick winners based on the polls. But the guessing is made a bit easier by what history tells us about the behavior of undecided voters, whose last-minute decisions can make all the difference.

Take the Senate race in Colorado, where Republican challenger Ken Buck either holds a slim lead or is in a dead heat, depending on the survey. The fact that he is neck-and-neck with an incumbent, Democratic Sen. Michael Bennet, actually is a reason for him to be optimistic, because of how undecided voters tend to break at the end.

Meanwhile, in California, Democratic Sen. Barbara Boxer, who has a single-digit lead in her re-election fight, would likely be home free if she were a new face to voters. But because she is seeking a fourth term, Sen. Boxer isn't completely out of the woods.

Why? Simply put, the rule of thumb is that incumbents get relatively few of the votes from those who say they are undecided on the eve of an election. So incumbents trailing at this late stage are in trouble. But so are incumbents with small leads or who are dead even in polls that put them only in the mid-40% range.

The theory isn't absolute, however. Like all rules in politics, it comes with enough caveats to give pollsters cover.

The basic idea is this:

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Re-election campaigns are almost always about the incumbent, and whether he or she deserves another term. Challengers are almost always less well-known. A campaign, especially for federal office or governor, tends to be long, heated and omnipresent on voters' television screens.

Amid all that exposure, the thinking goes, if a voter isn't persuaded to back the incumbent, it isn't likely he or she will do so at the campaign's end. And history shows many incumbents who are even in late polls lose because of that.

But reading this rule into end-of-the-road polls is tricky. First, and perhaps most important, the pollster trying to tally the size of the undecided pool must first push "leaners"—voters who lean toward one side but aren't committed—into one camp or the other. Otherwise, the undecided category would be unrealistically big, including people who aren't truly undecided. If fewer than, say, 85% to 90% of those polled back a candidate, the undecided pool may be too large.

Moreover, the rule about undecideds breaking against an incumbent holds more true if the candidate is a long-term incumbent. Those who were appointed to fill the job, and shorter-term incumbents, such as Sen. Bennet in Colorado, are less at risk.

Another factor of political life is that many of those who say they are undecided—even if they seem to be likely voters—don't vote. There is little research to quantify this, butThe pollsters' rule is that a quarter to a third of voters undecided in the last poll probably won't vote.

Finally, it's simply wrong to assume that an incumbent below 50% in the final poll will lose because he or she won't get any of the undecided voters. The incumbent will get some of them; the key is how close to a majority the incumbent is, andhow small a share of undecided voters is needed to hit the magic number.for victory.

The rule is in play in some key races this year. In Nevada, Senate Majority Leader Harry Reid has been in office 24 years. He and newcomer Sharron Angle are in a dead heat, or she has a small lead, depending on the poll. But with up to 10% of voters saying they are undecided, the smart money is on the challenger, because of the belief that more undecideds will wind up in Ms. Angle's camp than in Sen. Reid's.

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In Washington state, Democrat Patty Murray, elected to the Senate in 1994, is in slightly better shape than is Sen. Reid. She has a small lead in polls but only about 5% of the electorate is undecided. That means there are fewer votes likely to follow the anti-incumbent rule.

The dynamic exists in governor's races. Democratic Gov. Ted Strickland of Ohio trails by low- to mid-single digits, but is unable to exceed the mid-40% level in most surveys. Whichever poll is right, he'll need to win the undecideds. As history says, good luck with that.

—Peter A. Brown is assistant director of the Quinnipiac University Polling Institute and a regular columnist on WSJ.com.

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